The present invention relates to threaded pipe connections particularly useful in the oil and gas industry. In particular, the invention relates to a threaded pipe connection having high torque capacity.
High torque capacity is a desired feature in threaded tubular connections that are used in the oil and gas industry. The following are examples of some applications that require connections having high torque capacity.
During the cementing operations such as when setting a liner, operators will often reciprocate the liner axially, while also rotating the liner to produce a better cementing job. To do this, the liner connection must have high torque capacity; otherwise the pin may continue to advance into the box beyond the predetermined axial distance and thus ruin the connection. Workover strings are another example. Many times a tubing workover string is used to drill a small amount of cement. Drilling imparts additional torsional loads on the tubing connections. Again, without sufficient torque capacity, the pin will advance into the box beyond the desired point. There are several threaded tubular connections that are designed specifically for workover operations that require high torque capacity.
Drill pipe tool joints must also have a high torque capacity to perform their job. A tool joint that is not torqued properly or does not have high torque capacity will fail prematurely resulting in lost rig time and an expensive fishing job.
If production tubing connections are wedged correctly and are made up with adequate torque, then the connections will resist backing off when subjected to cyclic loading caused by heating and cooling during production and shut in operations. Flush-joint wash pipe and other types of completion tools, or their components, may also require high torque strength connections for resisting cyclic and/or operational loads.
High torque capacity or strength is inherently built into the design of some connections. Generally, this is accomplished in several ways such as having a connection with a pin having a diameter that is slightly larger than the mating diameter on the box at a certain axial distance, that is to say, when the pin is rotated into the box, it will show a stand off at the hand tight position. This stand off will be eliminated when the pin is made-up into the box by applying adequate torque to the connection. Additional torque capacity is added to the connection by machining strong external or internal shoulders or a combination of both. The shoulders will stop the pin from axially advancing when higher torque is applied. Connections of this type may have trapping angles on the shoulders for containment and for added strength. Some connections depend on a wedge thread design. Others will have a tool joint made out of higher strength material than that of the pipe itself, the tool joint then is threaded or welded into the pipe.
According to one embodiment of the present invention, a pipe connection having a box member and a pin member with interengageable threads is provided having a first thread which generates hoop tension in the box member and hoop compression in the pin member during make-up of the connection and a second thread adjacent the first thread which generates hoop compression in the box member and hoop tension in the pin member during make-up of the connection. The second thread may have a generally dove-tail shape or may be a hooked thread with a negative load angle. The hooked thread may also have a positive stab angle to facilitate stabbing of the connection. The first thread may be selected from a buttress thread, a round thread, an ACME thread or any other conventional oilfield thread. The first and/or second thread on the pin member may be configured to generate axial wedging with the mating, interengageable thread on the box member. The opposing hoop stresses generated by the first and second threads reduce the combined hoop stress acting on the connection.
One embodiment of the present invention is directed to a threaded pipe connection having box and pin members comprising interengageable thread means on the box and pin members, the thread means including a dove-tail thread having negative flank angles extending along the lengths of the box and pin members and an intermediate thread having positive flank angles extending along the lengths of the box and pin members adjacent the dove-tail thread, wherein the intermediate threads generate hoop tension in the box member and hoop compression in the pin member, and the dove-tail threads generate hoop compression in the box member and hoop tension in the pin member during engagement of the pin member with the box member. Torque is resisted by the radial interference and the opposing forces generated by the two different types of threads on the connection.
Another embodiment of the present invention is directed to a threaded pipe connection comprising a dual threaded pin member having a tapered, external generally dove-tail shaped thread having negative flank angles and a tapered, external intermediate thread having positive flank angles adjacent to the generally dove-tail shaped thread. The connection also has a dual threaded box member having a tapered, internal generally dove-tail shaped mating thread engageable with the dove-tail shaped thread of the pin member and a tapered, internal intermediate mating thread engageable with the intermediate thread of the pin member so that the respective generally dove-tail shaped thread and intermediate thread of the pin member engage the respective mating threads of the box member during rotational makeup of the connection.